For 1280x1024 you should use for example the following Metamode: "1280x1024, 640x480@1280x1024".
The GF4 MX contains a builtin tvout chipset. It is something developed by nvidia. The part can do max 1024x768 on TV's. Only 640x480 and 800x600 can be recommended on PAL and on NTSC only 640x480. All other resolutions are interlaced.